1. Field of the Invention
The present invention relates generally to communication systems employing multi-dimensional codes and, more particularly, provides an equalizer/decoder for adaptive equalization of one or more physical channels transmitting a plurality of symbols which must be decoded as a group but which, due to the number of symbols and available number of physical channels, are transmitted over multiple symbol intervals.
2. Description of Related Art
A generalized communication system includes a transmitter which transmits symbols to a receiver through a communication "channel." Communication systems have been designed to transmit through a wide variety of channels, including conductive metals such as copper, magnetic storage read/write channels, optical fiber, the atmosphere, water, and so on.
There are many modulation techniques which can benefit from the present invention. For background, however, one particularly well known approach is pulse amplitude modulation (PAM). In a PAM-based system, the data symbols are encoded into a sequence of electrical signals, or pulses, having two or more voltage levels. All channels tend to introduce noise and, of particular significance to PAM, introduce amplitude and phase distortion that broadens the pulses and causes intersymbol interference (ISI). The resulting distortion that leads a particular pulse and interferes with past pulses is called precursor ISI and the distortion that follows a particular pulse and interferes with future pulses is called postcursor ISI.
The typical receiver includes an "equalizer" which compensates the electric pulses for the channel distortion. In other words, the equalizer attempts to cancel or remove the ISI. A good overview of an equalizer in a single channel system can be found in my earlier U.S. Pat. No. 5,539,774, entitled "Dual Decision Equalization Method and Device," issued Jul. 23, 1996 and assigned to International Business Machines, Corporation. As explained therein, an equalizer can be linear or nonlinear and can be "trained" or "blind."
Nonlinear equalization, such as decision feedback equalization (DFE), was conventionally used to eliminate postcursor ISI from the present symbol due to previously transmitted and assigned symbols. However, since the decision device is inside the feedback loop the nonlinear DFE is subject to error propagation. Linear equalization does not involve any feedback of the output from the decision device and does not, therefore, suffer from error propagation. For the same reason, however, a linear equalizer does not have an exact knowledge of the previously assigned symbols and, therefore, can only estimate the postcursor ISI. In addition, the linear equalizer has the property of enhancing noise. In the '774 patent, I combined a linear decision-directed equalizer (DDE) with a modified nonlinear decision feedback equalizer (DFE) in order to obtain the benefits, but not the shortcomings in each type of equalizer.
More specifically, the '774 patent proposes a "trainer" system which communicates with a "trainee" system. The terms trainer and trainee were selected because the preferred embodiment implemented a blind equalization scheme that did not require a training sequence. The trainer system comprised a linear decision directed equalizer (DDE) having a feed forward filter and a decision device. The trainee system looked like a conventional decision feedback equalizer having a feed forward filter, a decision device, and a "feedback" filter with the exception that the feedback filter was modified to receive its input from the trainer system rather than the trainee system's decision device.
The '774 Patent is limited in scope because it only addressed "dual decision equalization" in the context of a single channel transmitting a sequence of symbols that are sliced and decoded in a strict symbol-by-symbol sequence. The '774 Patent, therefore, only teaches "dual decision equalization" wherein one symbol from a single symbol sequence is decoded at each symbol interval.
At very high data rates such as that required by the presently evolving standard required by gigabit Ethernet (e.g. a billion bits per second), it is desirable to use a coding scheme which encodes a plurality of data bits into a multi-dimensional constellation of data symbols that provides sufficient redundancy, coding gain and noise immunity. In the context of gigabit Ethernet, for example, the goal is to be able to transmit the electric signals over standard Category 5 unshielded twisted pair (UTP5). UTP5 has eight conductors provided as four twisted pairs which form four parallel communication channels. One potentially desirable coding scheme is a twenty-four dimensional (24D) lattice coding scheme. Assuming that the four pairs in the category 5 UTP carry a signal that is modulated in only one dimension (e.g. a five level PAM signal), it would be necessary to transmit and receive symbols over six baud intervals before the 24D constellation were fully defined.
Unfortunately, however, the equalization apparatus of the '774 Patent does not work with multiple channels (such as the four pairs in UTP) or with a symbol constellation that must be assembled over multiple baud intervals (e.g. six).
There remains a need, therefore, for a communication system which can transmit, receive and equalize a multi-dimensional code that is transmitted over multiple symbol intervals on one or more channels and for a combined equalizer and decoder which can handle multiple symbols received over multiple channels and multiple symbol periods.